Dual-slab-layer low-loss silicon optical modulator

ABSTRACT

A silicon optical modulator is fabricated to have a multi-slab structure between the contacts and the waveguide, imparting desirable performance attributes. A first slab comprises dopant of a first level. A second slab adjacent to (e.g., on top of) the first slab, comprises a doped region proximate to a contact, and an intrinsic region proximate to the waveguide. The parallel resistance properties and low overlap between the highly doped silicon and optical mode pigtail afforded by the multi-slab configuration, allow the modulator to operate with reduced optical losses and at a high speed. Embodiments may be implemented in a Mach-Zehnder interferometer or in micro-ring resonator modulator configuration.

BACKGROUND

The present invention relates to optical communication techniques. Moreparticularly, the present invention provides a silicon opticalmodulator.

Over the last few decades, the use of communication networks hasexploded. In the early days Internet, popular applications were limitedto emails, bulletin board, and mostly informational and text-based webpage surfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. With such high demands on data and data transfer,existing data communication systems need to be improved to address theseneeds.

Progress in computer technology (and the continuation of Moore's Law) isbecoming increasingly dependent on faster data transfer between andwithin microchips. Optical interconnects may provide a way forward, andsilicon photonics may prove particularly useful, once integrated on thestandard silicon chips. 40-Gbit/s and then 100-Gbit/s data rates WDMoptical transmission over existing single-mode fiber is a target for thenext generation of fiber-optic communication networks.

SUMMARY

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides a low-loss siliconoptical modulator that is directly integrated in a silicon photonicschip for high data rate DWDM optical communications, though otherapplications are possible.

A silicon optical modulator is fabricated to have a multi-slab structurebetween the contacts and the waveguide, imparting desirable performanceattributes. A first slab comprises dopant of a first level. A secondslab adjacent to (e.g., on top of) the first slab, comprises a dopedregion proximate to a contact, and an intrinsic region proximate to thewaveguide. The parallel resistance properties and low overlap betweenthe highly doped silicon and optical mode pigtail afforded by themulti-slab configuration, allow the modulator to operate with reducedoptical losses and at a high speed. Embodiments may be implemented in aMach-Zehnder interferometer or in micro-ring resonator modulatorconfiguration.

An optical modulator according to an embodiment comprises a waveguidehaving a first waveguide portion of a first conductivity type and asecond waveguide portion of a second conductivity type opposite to thefirst conductivity type, the waveguide configured to communicate anoptical signal along a first axis defined within a silicon substrate. Afirst contact of the first conductivity type is proximate to the firstwaveguide portion. A second contact of the second conductivity type isproximate to the second waveguide portion. A first multi-slab structureoffers parallel conductive pathways of different resistance between thefirst contact and the first portion. A second multi-slab structureoffers parallel conductive pathways of different resistance between thesecond contact and the second portion. A first slab of the firstmulti-slab structure proximate to the silicon substrate, is adjacent toa second slab of the first multi-slab structure distal from the siliconsubstrate, along a second axis orthogonal to the first.

An embodiment of a method comprises creating a first conductive pathwayof a first resistance between a first contact of a first conductivitytype and a first waveguide portion of the first conductivity type. Asecond conductive pathway of a second resistance lower than the firstresistance is created between the first contact and the first waveguideportion, the second conductive pathway parallel to the first conductivepathway. A third conductive pathway of a third first resistance iscreated between a second contact of a second conductivity type oppositeto the first conductivity type, and a second waveguide portion of thesecond conductivity type. A fourth conductive pathway of a fourthresistance lower than the third resistance, is created between thesecond contact and the second waveguide portion, the fourth conductivepathway parallel to the third conductive pathway. A potential differenceis applied between the first contact and the second contact toreverse-bias a junction formed at an interface of the first waveguideportion and the second waveguide portion.

An optical modulator integrated with a silicon photonics systemcomprises a first phase-shifter on a silicon substrate, and a secondphase-shifter on the silicon substrate. A first 2×2 splitter has a firstexit port coupled to an input port of the first phase-shifter and asecond exit port coupled to an input port of the second phase-shifter. Asecond 2×2 splitter has a first entry port coupled to an output port ofthe first phase-shifter and a second entry port coupled to an outputport of the second phase-shifter. The first phase-shifter comprises afirst contact of the first conductivity type proximate to the firstwaveguide portion, and a second contact of the second conductivity typeproximate to the second waveguide portion. The first phase-shifterfurther comprises a first multi-slab structure offering parallelconductive pathways of different resistance between the first contactand the first portion, and a second multi-slab structure offeringparallel conductive pathways of different resistance between the secondcontact and the second portion. A first slab of the first multi-slabstructure proximate to the silicon substrate, is adjacent to a secondslab of the first multi-slab structure distal from the siliconsubstrate, along a second axis orthogonal to the first axis.

Various embodiments achieve these benefits and others in the context ofknown silicon waveguide laser communication technology. However, afurther understanding of the nature and advantages of the presentinvention may be realized by reference to the latter portions of thespecification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is a schematic diagram of an optical modulator based upon a phaseshift structure formed on silicon according to an embodiment.

FIG. 2 is a simplified cross-sectional diagram of a dual-slab phaseshift structure according to an embodiment.

FIG. 3 shows a corresponding circuit for the dual-slab phase shiftstructure of FIG. 2.

FIG. 4 is a cross-sectional plot of optical intensity for dual-slabphase shift structure of FIG. 2.

FIG. 5 is flow chart showing a method of performing low-loss modulationof an optical signal in a compact device integrated in a system-on-chipaccording to an embodiment.

DESCRIPTION

The present invention relates to optical telecommunication techniques.More particularly, the present invention provides a low-loss opticalmodulator formed directly on a silicon substrate, though otherapplications are possible.

In modern electrical interconnect systems, high-speed serial links havereplaced parallel data buses, and serial link speed is rapidlyincreasing due to the evolution of CMOS technology. Internet bandwidthdoubles almost every two years following Moore's Law. But Moore's Law iscoming to an end in the next decade. Standard CMOS silicon transistorswill stop scaling around 5 nm. And the internet bandwidth increasing dueto process scaling will plateau. But Internet and mobile applicationscontinuously demand a huge amount of bandwidth for transferring photo,video, music, and other multimedia files. This disclosure describestechniques and methods to improve the communication bandwidth beyondMoore's law.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, entry, exit, clockwise and counter clockwise have beenused for convenience purposes only and are not intended to imply anyparticular fixed direction. Instead, they are used to reflect relativelocations and/or directions between various portions of an object.

FIG. 1 is a schematic diagram of a low-loss optical modulator formed ona silicon substrate according to an embodiment of the present invention.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications.

As shown, the optical modulator 100 is provided formed on a siliconsubstrate. Optionally, the silicon substrate is a silicon-on-insulatorsubstrate.

The optical modulator 100 is a device fully integrated in a compact formin a chip. In the embodiment, the optical modulator 100 includes a firstphase-shifter 131 coupled a second phase-shifter 132 optically inparallel.

At an input side, a first 2×2 splitter 111 is coupled to an input portof each of the first phase-shifter 131 and the second phase-shifter 132.At an output side, a second 2×2 splitter 112 is coupled to an outputport of each of the first phase-shifter 131 and the second phase-shifter132.

An entry port of the first 2×2 splitter 111 is configured to couple withan input fiber, which needs modulation for an optical signal that hasalready been transported through the input fiber. An exit port of thesecond 2×2 splitter 112 is configured to couple with an output fiber foroutputting the optical signal after the modulation.

Optionally, the 2×2 splitter is a multimode interference coupler.Optionally, the 2×2 splitter is a directional coupler.

Optionally the first phase-shifter 131 is a first waveguide formed onthe silicon substrate and the second phase-shifter 132 is a secondwaveguide formed on the same silicon substrate.

Optionally, the second waveguide coupled with the first waveguide inparallel with a relative phase delay to form a Mach-Zehnderinterferometer.

Optionally, the first waveguide 131 comprises a first core material witha first index of refraction n₁ and an elongated shape of the firstlength L₁ embedded in a first cladding material 141 on the siliconsubstrate. In a cross-section view, an example of a waveguide is shownwith a core material in a typical rectangular shape embedded in acladding material overlying a substrate.

The cladding material usually has an index of refraction smaller thanthat of the core material so that the light can be confinedsubstantially inside the geometry of the core of the waveguide. Acombination effect of the waveguide core with a certain geometric shapeand respective dimensions and the corresponding indices of refractionfor both the first core material and the first cladding material, yieldsa first phase delay for the optical signal of the certain wavelengthpassing the first phase-shifter 131.

Similarly, the second waveguide 132 includes a second core material witha second index of refraction n₂ and an elongated shape of the secondlength L₂ embedded in the first cladding material 141 formed on the samesilicon substrate. A second phase delay for the optical signal passingthe second phase-shifter 132 is yielded. The second phase delay may bedifferent from the first phase delay.

Optionally, the first waveguide and the second waveguide coupled inparallel with a relative phase delay to form a Mach-Zehnderinterferometer. Overall physical length of the Mach-Zehnderinterferometer including both 2×2 splitters 111 and 112 can be madequite compact. For example, the length of the modulator 100 can be justabout 100 μm.

In an embodiment, a total phase delay of the modulator 100 is amanifestation of the first phase delay and the second phase delayassociated with the structure provided by the Mach-Zehnderinterferometer formed by the first waveguide 131 and the secondwaveguide 132 coupling to the Mach-Zehnder interferometer.

Each of the first waveguide and the second waveguide can beindependently fabricated and tuned with material and geometryselections. Optionally, a heating element can be installed to be aroundeach waveguide to tune the index of refraction by changing temperature.

As a result of tuning the first waveguide and the second waveguide aswell as properly selecting the corresponding first and second claddingmaterials under the structure described herein, low loss modulation canbe achieved. The modulator 100 is directly integrated in a silicon chipwith compact dimensions.

In some embodiments, tuning the first waveguide and the second waveguidecan done both during their fabrication and afterward. Optionally, duringthe fabrication, the core material and cladding material of eachwaveguide can be properly selected for achieving different value of thephase delay. Optionally, the core material of each waveguide can be oneselected from single crystal silicon, poly-crystal silicon, SiN, Si₃N₄,SiON, silicon germanium alloy (Si_(x)Ge_(1-x)), or other materialscompatible with a silicon-on-insulator substrate.

Optionally, the core material for the first waveguide 131 and the secondwaveguide 132 is the same. Optionally, the core material for the firstwaveguide 131 is different from that for the second waveguide 132. Thefirst cladding material 141 can be one selected from SiO₂, SiN, Si₃N₄,SiON, Air, silicon germanium alloy (Si_(x)Ge_(1-x)), and indium tinoxide.

In some embodiments, the core of each waveguide can be formed variablywith a length and cross-section shape. Optionally, the core can beformed with a cross-sectional shape in rectangular, a simple channelwaveguide. Or the core can be formed in complex shape such as acombination of two rectangular shapes. For example, a rib waveguidehaving a smaller rectangle on top of a wider rectangle, a slot-channelwaveguide having two rectangles in parallel separated by a small gap, aslot-rib waveguide having a slot-channel on top of a wider rectangle,and a multi-channel waveguide having two rectangles stacking together.In another example, a triangle shape waveguide can be provided.Optionally, different structures can be designed for the first waveguide131 and the second waveguide 132 in order to realize different effectiveindice of refraction.

Each phase-shifter is a waveguide formed with a core of a certain shapeand a length within a cladding, which can be characterized by aneffective index of refraction n_(eff) and a length L. The effectiveindex of refraction of the phase-shifter depends on the shape the core,the indices of refraction of both the core material and the claddingmaterial, and other properties such as optical-thermal coefficient, modestructure associated with the geometry, wavelength and polarization modeof signals, etc.

In some embodiments, a unit of the first phase-shifter 131 and thesecond phase-shifter 132 including at least the first 2×2 splitter 111can be used as a duplicate phase-shifter unit. The duplicatephase-shifter unit can be cascaded in series multiple times.

According to embodiments, a phase change component of a silicon opticalmodulator is fabricated to have a multi-slab structure between thecontacts and the waveguide, imparting desirable performance attributes.A first slab comprises dopant of a first level. A second slab adjacentto (e.g., on top of) the first slab, comprises a doped region proximateto a contact, and an intrinsic region proximate to the waveguide. Theparallel resistance properties afforded by the multi-slab configuration,allow the modulator to operate with reduced losses and at a high speed.Embodiments may be implemented in a Mach-Zehnder interferometer or inmicro-ring resonator modulator configuration.

It is again noted that future optical communication system is driven bythe increasing demand of higher data bandwidth and lower cost. Siliconphotonics integration offers a solution, given its enhanced performanceand compatibility with CMOS fabrication processes.

High speed Optical signals travelling between data centers need siliconoptical modulator with high working bandwidth and low optical loss.Three indicators can determine the performance of the silicon opticalmodulator:

-   -   modulation efficiency;    -   working bandwidth; and    -   optical loss.

Conventionally, improvement of any one indicator may be achieved at theexpense of reducing the performance of another indicator. However,embodiments provide a silicon optical modulator exhibiting lower opticalloss, while maintaining working bandwidth and modulation efficiency.

Certain modulators may be based upon the electro-optical (EO) effect insilicon. This EO effect refers to changes in optical properties of amaterial in response to an electric field that varies slowly as comparedwith the frequency of light.

In silicon, the EO effect usually relates to the change of opticalrefractive index (n) and absorption coefficient (a). There are two kindsof EO effect in silicon:

-   -   the direct EO effect, and    -   the indirect EO effect.

As silicon has a symmetric atomic structure, there is no 1^(st) orderdirect EO effect in silicon. And, the 2nd order direct EO effect (Kerreffect) is weak.

Accordingly, a silicon modulator may be designed based upon the indirectEO effect in silicon. That indirect EO effect is known as thefree-carrier dispersion effect.

A working principle of a silicon optical modulator operating accordingto the free-carrier dispersion effect, is now described. Underfree-carrier dispersion effect, the refractive index and absorptioncoefficient of doped silicon can be changed, together with theconcentration of free electron and holes which can be tuned by anexternally applied electrical field.

Usually, the changing of free-carrier concentration is realized byimplanting a P-N junction in the center of the silicon waveguide. Thus,when an optical wave is traveling through the waveguide, opticalparameters such as optical phase and optical intensity can be tuned bythe external electrical field.

Optical and electrical structures of a silicon optical modulator are nowdescribed. In particular, changing of absorption coefficient withinsilicon may not be efficiently achieved. Thus, in order to change therefractive index of silicon to accomplish the desired modulation, someoptical structures may be needed to convert the optical phase change toan optical intensity change.

The possible optical structures for realizing this goal, areMach-Zehnder interferometers and micro-ring resonators. And, anelectrical structure for achieving this effect, is a reversed PNjunction.

Accordingly, FIG. 2 shows a simplified cross-sectional diagramillustrating a phase shift structure 200 according to an embodiment.This phase shift structure comprises a waveguide 202 comprising a Pdoped portion 204 and an N doped portion 206.

A P/N junction (j) 208 is present between the P doped portion and the Ndoped portion. Application of a reverse bias across the junction 208,can change the concentration profile of the free carriers on the crosssection of the waveguide, and hence the refractive index and opticalphase of the waveguide. The optical structure such as a Mach-Zehnderinterferometer shown in FIG. 1 achieve the conversion of an opticalphase change to an optical intensity change and hence achieve thedesired modulation of an optical signal traveling along the waveguide.

In particular, phase change structure 200 further comprises heavilydoped P++ contact region 210, and heavily doped N++ contact region 212.These contact regions are in electrical contact with a source ofpotential difference in order apply a reverse bias to the junction.

Application of the reverse bias across the P/N junction of thewaveguide, serves to control the phase difference (ϕ)) achieved by thephase change structure. As shown below, this effect is based upon adelta in n and p type character of the material within the waveguide:

-   -   Δϕ(Δp, Δn)    -   Δn_(eff)(Δp, Δn);    -   Δα(Δp, Δn).

According to embodiments, the electrical potential applied to thecontacts is communicated to the portions of the waveguide, utilizing atwo-slab structure. On the P side, a first slab 214 (here located on thebottom) is doped to a uniform level or non-uniform level. On the N side,a first slab 216 (again, located on the bottom) is also doped to auniform level or non-uniform level.

A second slab (here located on the top) comprises a doped regionproximate to the contact, and an intrinsic (i) region proximate to thewaveguide. Thus, on the P side, the second slab 218 comprises a P+region 220, and an adjacent intrinsic region 222. On the N side, thesecond slab 224 comprises a N+ region 226 and an adjacent intrinsicregion 228.

A variety of different techniques can be utilized to form intrinsicportions as shown in the FIG. 2. According to one approach, dopant canbe implanted with sufficient energy to pass through the intrinsicportion, and a reversed type of dopant can be used to compensate theprevious dopant then reside below the intrinsic portion in the lowerslab layer. Alternatively, the intrinsic portion may comprise a separateintrinsic layer formed after/above the lower slab layer, withlithography utilized to exclude subsequent doping from only theintrinsic portion (but not the adjacent doped portion of the upper slablayer). Thus two possible methods to realize embodiments include:compensating the first type dopant with a second implantation withreversed type of dopant on the top of the slab, and using a lithographicmask and implantation condition to tune the doping profile and result inthe intrinsic layer.

This resulting dual-slab architecture imparts desired properties to thephase shift structure. In particular, the dual-slab architecture offerslow-loss performance, coupled with high speed operation.

In particular, FIG. 3 shows a corresponding circuit 300 for thedual-slab phase shift structure of FIG. 2. Specifically, FIG. 3 showsthe parallel resistance paths 302 afforded by the multi-slab structure.

Here, the low loss characteristic is achieved by the low-level of dopingon the top slab:

-   -   R_sp_top>R_sp_bot (p-doped side)    -   R_sn_top>R_sn_bot (n-doped side).

The higher resistance of the top slab may result from a combination ofan absolute lower doping level in the top slab relative to the bottomslab, and also the presence of the intrinsic region in the top slab.

Lower doping level on the top of the slab may primarily be used toachieve a lower optical loss, as the optical intensity is strong on thetop slab layer. This is shown in FIG. 4.

Meanwhile, reducing the doping level on the top slab will induce a lowerspeed operation. So utilizing a higher doping level on the bottom slabcan compensate the speed performance.

Given a carefully controlled design of the doping levels, the speedperformance can be higher than with a modulator architecture featuringonly a single slab. In this manner, the high speed operation of thedual-slab layer design offers a benefit.

In summary, the dual-slab structure helps to fine tune the dopingprofile of the modulator in order to achieve a lower optical loss andhigher speed simultaneously. By contrast, in a conventional one-slabmodulator structure, the two performance indicators of optical loss andoperational speed are a trade-off between one another.

High speed operation of the modulator is achieved by the high-leveldoping on the bottom slab layer:

-   -   R_sp_top//R_sp_bot<R_sp    -   R_sn_top//R_sn_bot<R_sn, where:        -   R_sp=resistance of a one-slab structure; and        -   R_sn=resistance of a one-slab structure.

In this manner, the intrinsic portion serves to reduce the optical loss,while reducing the speed. However, the speed performance is compensatedfor by increasing the doping level on the bottom slab. Here,R_sp_top//R_sp_bot<R_sp and R_sn_top//R_sn_bot<R_sn, ensures that thedual-slab structure has a higher speed than would a conventionalone-slab structure.

FIG. 4 is a cross-sectional plot of optical intensity for dual-slabphase shift structure of FIG. 2. This plot shows that the top slab layerhas higher optical intensity.

In particular, optical intensity is strong on the top of the slab, andweaker on the bottom slab. When the doping level on the top slab isreduced, and as the intrinsic layer close to the corner between the slaband the waveguide (called the rib or ridge), the optical loss reduces atthat corner because reducing the doping level actually reduce the freecarriers' absorption. But, when the doping level is reduced on the topof the slab, if everything else is maintained the same, the totalresistance is increased and the speed will be lower.

To maintain the speed, it is seen that the optical intensity on thebottom of the slab is weak. If the doping level at the bottom of theslab is increased until the total resistance is the same or even lowerthan the case that without the intrinsic layer on the top slab, thenspeed can be maintained or even increased.

While the optical loss from the bottom slab will be increase somewhat,its contribution to the overall loss will not be increased that muchbecause the optical intensity is much weaker as compared to the top slablayer. So with the advantage of the different optical intensity on thetop and bottom of the slab, a window can be found to overcome thetrade-off between optical loss and speed.

For purposes of further illustration, consider a hypothetical case wherethere is no optical intensity on the bottom slab, and all of the opticalintensity lies on the top slab. Under such circumstances, the bottomslab can have an extremely high doping. That doping level is high enoughto produce a very low total resistance and hence an extreme high speed.At the same time, the contribution on the optical loss is zero, as thereis no optical intensity at the bottom slab. In this manner the trade-offbetween optical loss and operational speed is overcome by the dual-slabarchitecture, and these performance characteristics may be decoupled.

While the above description has focused upon an optical modulatorarchitecture comprising two slabs, which offers a simplest case from aprocessing perspective. However, embodiments are not limited to only twoslabs, and could instead feature more than two slabs. Modulatorarchitectures featuring more than two slab layers could help to tailorthe doping profile to more precisely match the desired optical intensityprofile, resulting in optimum performance of the optical modulator.

And while the above description has focused upon a dual-slab opticalmodulator architecture having an intrinsic portion in the top slab, thisis also not required. Alternative embodiments could provide an intrinsicportion in the bottom slab instead.

Embodiments may offer benefits to reduce the gap of implementation ofsilicon photonics integration circuits over other types, so that thedata center communication can enjoy more of the low-cost advantageconferred by CMOS process based silicon photonics.

FIG. 5 is flow chart showing a method 500 of performing low-lossmodulation of an optical signal in a compact device integrated in asystem-on-chip according to an embodiment. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications.

At 502 a first conductive pathway of a first resistance is createdbetween a first contact of a first conductivity type and a firstwaveguide portion of the first conductivity type.

At 504, a second conductive pathway of a second resistance lower thanthe first resistance, is created between the first contact and the firstwaveguide portion. The second conductive pathway is parallel to thefirst conductive pathway.

At 506, a third conductive pathway of a third first resistance iscreated between a second contact of a second conductivity type oppositeto the first conductivity type, and a second waveguide portion of thesecond conductivity type;

At 508, a fourth conductive pathway of a fourth resistance lower thanthe third resistance, is created between the second contact and thesecond waveguide portion. The fourth conductive pathway is parallel tothe third conductive pathway.

At 510, a potential difference is applied between the first contact andthe second contact to reverse-bias a junction formed at an interface ofthe first waveguide portion and the second waveguide portion.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. An optical modulator comprising: a waveguidehaving a first waveguide portion of a first conductivity type and asecond waveguide portion of a second conductivity type opposite to thefirst conductivity type, the waveguide configured to communicate anoptical signal along a first axis defined within a silicon substrate; afirst contact of the first conductivity type proximate to the firstwaveguide portion; a second contact of the second conductivity typeproximate to the second waveguide portion; a first multi-slab structureoffering parallel conductive pathways of different resistance betweenthe first contact and the first portion; and a second multi-slabstructure offering parallel conductive pathways of different resistancebetween the second contact and the second portion, wherein: a first slabof the first multi-slab structure proximate to the silicon substrate, isadjacent to a second slab of the first multi-slab structure distal fromthe silicon substrate, along a second axis orthogonal to the first axis.2. An optical modulator as in claim 1 wherein the first slab exhibits aresistance lower than the second slab.
 3. An optical modulator as inclaim 2 wherein the first slab has a higher doping of the firstconductivity type than the second slab.
 4. An optical modulator as inclaim 3 wherein the second slab comprises: an intrinsic portionproximate to the first waveguide portion, and a doped portion proximateto the first contact.
 5. An optical modulator as in claim 1 comprising aMach-Zehnder interferometer.
 6. An optical modulator as in claim 1comprising a micro-ring resonator.
 7. An optical modulator as in claim 1wherein the silicon substrate comprises a silicon-on-insulator (SOI)substrate.
 8. An optical modulator as in claim 1 wherein the second slabexhibits a resistance lower than the first slab.
 9. A method comprising:creating a first conductive pathway of a first resistance between afirst contact of a first conductivity type and a first waveguide portionof the first conductivity type; creating a second conductive pathway ofa second resistance lower than the first resistance, between the firstcontact and the first waveguide portion, the second conductive pathwayparallel to the first conductive pathway; creating a third conductivepathway of a third first resistance between a second contact of a secondconductivity type opposite to the first conductivity type, and a secondwaveguide portion of the second conductivity type; creating a fourthconductive pathway of a fourth resistance lower than the thirdresistance, between the second contact and the second waveguide portion,the fourth conductive pathway parallel to the third conductive pathway;and applying a potential difference between the first contact and thesecond contact to reverse-bias a junction formed at an interface of thefirst waveguide portion and the second waveguide portion.
 10. A methodas in claim 9 further comprising communicating an optical signal throughthe first waveguide portion and the second waveguide portion along afirst axis defined in a silicon substrate, wherein the second conductivepathway and the fourth conductive pathway are proximate to the siliconsubstrate along a second axis orthogonal to the first axis, and thefirst conductive pathway and the third conductive pathway are distalfrom the silicon substrate along the second axis.
 11. A method as inclaim 10 wherein: the first conductive pathway comprises a firstintrinsic portion proximate to the first waveguide portion; and thethird conductive pathway comprises a second intrinsic portion proximateto the second waveguide portion.
 12. A method as in claim 9 wherein: thefirst conductive pathway comprises a first intrinsic portion proximateto the first waveguide portion; and the third conductive pathwaycomprises a second intrinsic portion proximate to the second waveguideportion.
 13. A method as in claim 9 wherein the first waveguide portionand the second waveguide portion are part of a Mach-Zehnderinterferometer.
 14. A method as in claim 9 wherein the first waveguideportion and the second waveguide portion are part of a micro-ringresonator.
 15. An optical modulator integrated with a silicon photonicssystem comprising: a first phase-shifter on a silicon substrate; asecond phase-shifter on the silicon substrate; a first 2×2 splitterhaving a first exit port coupled to an input port of the firstphase-shifter and a second exit port coupled to an input port of thesecond phase-shifter; and a second 2×2 splitter having a first entryport coupled to an output port of the first phase-shifter and a secondentry port coupled to an output port of the second phase-shifter,wherein the first phase-shifter comprises, a first contact of the firstconductivity type proximate to the first waveguide portion; a secondcontact of the second conductivity type proximate to the secondwaveguide portion; a first multi-slab structure offering parallelconductive pathways of different resistance between the first contactand the first portion; and a second multi-slab structure offeringparallel conductive pathways of different resistance between the secondcontact and the second portion, wherein: a first slab of the firstmulti-slab structure proximate to the silicon substrate, is adjacent toa second slab of the first multi-slab structure distal from the siliconsubstrate, along a second axis orthogonal to the first axis.
 16. Anoptical modulator as in claim 15 wherein the first slab exhibits aresistance lower than the second slab.
 17. An optical modulator as inclaim 16 wherein the first slab has a higher doping of the firstconductivity type than the second slab.
 18. An optical modulator as inclaim 17 wherein the second slab comprises: an intrinsic portionproximate to the first waveguide portion, and a doped portion proximateto the first contact.
 19. An optical modulator as in claim 15 comprisinga Mach-Zehnder interferometer.
 20. An optical modulator as in claim 15comprising a micro-ring resonator.